INSIGHTS | PERSPECTIVESsciencemag.org SCIENCEPHOTO: PETER SOROYEBy Jon Bridle and Alexandra van RensburgI
n 1949, environmentalist Aldo Leopold
wrote that “one of the penalties of an
ecological education is that one lives
alone in a world of wounds” ( 1 ). Seventy
years later, biologists no longer wit-
ness such wounds in solitude. Instead,
millions of people on social media share
evidence every day of how the behavior of a
wealthy minority ( 2 ) has created unsustain-
able rates of biodiversity loss and climate
transformation ( 3 ). Now, on
page 685 of this issue, Soroye
et al. demonstrate widespread
declines in bumble bee species
that are better explained by the
frequency of climate extremes
than by changes in average tem-
peratures ( 4 ).
Despite increasingly precise
predictions of rises in aver-
age temperatures and the fre-
quency of extreme weather
events, biologists still cannot
predict how ecological com-
munities will respond to these
changes. This means that scien-
tists cannot predict where, and
at what rates of climate change,
ecosystems will stop providing
the rainfall, decomposition,
and biological productivity on
which all economies depend.
Another key unknown is to
what extent ongoing habitat
and biodiversity loss reduces
the ability of ecological com-
munities to evolve in response
to the climate crisis ( 3 ).
To determine these critical
rates of biodiversity loss and
climate change as well as where they are
being exceeded ( 5 ), scientists test for shifts
in the distribution of species over time and
across their geographical ranges. Such stud-
ies reveal that the warming climate leaves a
footprint: The abundances of many plant,
animal, and fungal species have contracted
at low latitudes and elevations, and have
increased at high latitudes and elevations
( 6 ). How these responses to environmental
change vary according to species’ life histo-ries, ecologies, and their biotic interactions
provides a test of which ecosystems and lo-
calities are least resilient to global change.
Soroye et al. used long-term datasets to as-
sess changes in the abundance and geograph-
ical distribution of 66 bumble bee species in
Europe and North America between two pe-
riods, 1901–1974 and 2000–2014. Two of their
findings are especially alarming. Bumble
bee populations showed substantial declines
at southern (warming) ecological margins
but fewer compensating population expan-sions at northern (cooler) margins, suggest-
ing widespread declines in bee biodiversity
across both continents. Moreover, the causes
of these declines apparently depend more on
the frequency of extremely warm years than
on increases in average temperatures. As pre-
vailing temperatures climb closer to species’
physiological limits, extreme climate events
will become increasingly associated with bio-
diversity loss. In addition, their effects will
become more pronounced as cooler habi-
tats, where organisms can survive unusually
warm periods (e.g., deeper water, higher el-
evations), become increasingly rare.Shifts in species’ distributions in time and
space vary considerably across those taxa
and latitudes for which detailed data exist.
For example, the physiological thermal limits
of marine organisms tend to closely predict
their spatial distributions, whereas those of
terrestrial organisms do not. This is probably
because habitat loss and fragmentation limit
dispersal on land more than in the ocean ( 7 ).
Surprisingly, however, the new study shows
that bumble bee range expansions are just
as rare in less intensively farmed landscapes
as they are in intensively farmed
ones where habitat fragmenta-
tion is higher. Why range ex-
pansions in temperate bumble
bees are relatively rare, even
across relatively undisturbed
environments, demands further
investigation.
The ability of organisms to
alter their behavior or the tim-
ing of key life events such as
hibernation, flowering time,
and germination can minimize
organisms’ exposure to climate
extremes. Such plasticity can
slow population declines and
accelerate range expansions ( 8 ).
Also, many organisms threat-
ened by warming persist by
dispersing to locally cool micro-
climates ( 9 ). This active agency
of organisms to select suitable
habitats in time and space
tends to increase population
fragmentation at a fine spatial
scale while retaining occupancy
at larger spatial scales ( 10 ).
However, beyond a criti-
cal amount of environmental
change—arguably similar to
that routinely experienced during a species’
history—plasticity will no longer have suf-
ficient scope to buffer climate extremes ( 11 ).
As climates exceed these critical limits, the
widespread declines now observed for bum-
ble bee species will manifest in more and
more organisms and places. These declines
also will be increasingly associated with ex-
treme climatic events rather than average
changes in temperature ( 6 ).
Rapid evolution could also prevent de-
clines in population abundance and allow
range shifts despite habitat fragmentation
( 12 ). This might result from natural selec-ECOLOGYDiscovering the limits of ecological resilience
Bumble bee declines reveal species pushed to the edge of their environmental tolerances
School of Biological Sciences, University of Bristol, Bristol
BS8 1TH, UK. Email: [email protected]Declines in Bombus terricola landed them on Canada’s “Species at Risk” list. Climate
change seems to be the cause, with population declines better explained by more
frequent temperature extremes rather than by changes in average temperatures.626 7 FEBRUARY 2020 • VOL 367 ISSUE 6478
Published by AAAS